Peripheral Component Interconnect Express, PCIe 2.0 specifies 5.0 Gigbit/s symbol rate per lane. Multiple lanes can be used to fabricate larger port bandwidths. For example, x4 port would have an aggregate symbol rate of 20G, and a bit rate of 16G, 8b10b coding is used. A x8 port would have an aggregate symbol rate of 40G, and a bit rate of 32G. There are other serial interconnect protocols, for example serial rapid IO and Ethernet that have similar properties.
Referring to FIGS. 1 and 2, there are illustrated scheduler decision abstractions. The examples of FIGS. 1 and 2 are for scheduling a crossbar switch. The crossbar switch 10 includes input ports 12, output ports 14 and a switch fabric 16. Input ports 12 can be configures as x8 ports 18 and x4 ports 19. Output ports 14 can also be configured as x8 ports 20 and x4 ports 21. The spotted dots and checkered dots represent request connections on the white and black clock phase respectively. For crossbar scheduling only one dot per phase (color), per row and column. With FIFO queuing, only one request per row per phase (color). A logical port for a 1×8 port consists of two internal switch fabric (ISF) connections. Then dots 22 (spotted) represent requests for the white clock tick, while dots 24 (checkered) represent requests for the black clock tick. The dot 25 represents a request for both black and white clock ticks. The ellipses 26, 28 and 30 in the figures illustrate what a consistent decision would be. That is, in FIG. 1, ellipse 26 represents a x8 port to an x8 port connection, which preservers order and bandwidth of data switched. While ellipse 2 represents a x4 port to an x8 port connection, which also preservers order and bandwidth of data switched. Similarly, in FIG. 2, ellipse 30 represents a x8 port to a x4 port connection, which once again preservers order and bandwidth of data switched.
In order to simplify the ingress and egress queue management, it is desirable to make the scheduling decision such that the logical ports make consistent port selections.